Vegetable gardening can be highly rewarding; it can also be sorely disappointing. Torrential rains followed by scorching heat cause a host of problems, from blossom end rot to dropping blossoms. Diseases and insect pests ravage the garden, and let’s not forget the deer and the rabbits. But of all these challenges, success or failure in the vegetable garden is most determined by three factors: weather conditions, moisture, and soil quality. Of these three factors, soil is hands down the most important one. Fortunately, soil quality is also one area of gardening over which you have some control.
Quality soil is as important to plant health as quality food is to us. Gardeners often think of soil quality in terms of its fertility. If the soil isn’t fertile, then dump a bag of fertilizer on it and it’s good to go—or in this case, grow. But, just as downing a vitamin every morning won’t counteract a daily diet of Slim Jims and potato chips, adding fertilizer to poor soil won’t give you the results you’re looking for.
Good soil actually has three components: structure, fertility, and soil pH. Each component is important to the health of your plants.
Dig a hole in your vegetable garden, and you’ll notice that the soil has layers. The top layer, or topsoil, is usually darker, looser, and lighter than the subsoil, or bottom layer, which is often dense and hard. If you’re lucky, the topsoil is deep, light, and rock-free. If you’re like most gardeners, though, your topsoil is only a few inches deep and might be heavy or full of rocks.
Soil is made up of organic matter, mineral particles, air, and water. The composition of these particles determines the soil type and varies widely from region to region and perhaps, even within your own backyard. Soils are classified according to the size of particles, from sand to loam to silt to clay. Most soil types have some potential benefits and drawbacks.
Sandy soils have large particles. If you pick up a handful of sandy soil, it feels light, grainy, and dry. Sandy soils warm up early in the spring. They drain well and are rich in oxygen. However, water leaches out quickly, drying plants out and taking nutrients with it.
Clay soils are at the opposite end of the spectrum, with very small particles. If you pick up a handful of clay soil, it feels sticky or even forms a hard ball. Clay soils stay moist and are usually fertile. Unfortunately, they have little oxygen and stay cold until late spring. Plants germinate slowly in clay soils and might even rot.
Silty soil has particles sized somewhere in between those of sand and clay. It usually feels smooth and silky when you handle it. Although silt can be reasonably fertile, it can also be troublesome. When very wet, silt can retain too much water and easily turns into mud; when very dry, it can become dusty and blow away. Erosion is often a problem with extremely silty soil.
Loam soil, the Holy Grail of gardeners everywhere, is the ideal soil type for vegetable gardens. A handful of loam soil feels slightly moist, crumbly, and light. Loam soil is sometimes said to have the texture of chocolate cake or bread. It is easy to work but still holds moisture and nutrients. Plants grow well in it, and it’s a breeze to cultivate and work.
Like humans, plants need nutrients to grow and thrive. In addition to many micronutrients, or nutrients that are needed in small amounts, the three main nutrients gardeners worry about are nitrogen, phosphorus, and potassium. Nitrogen fuels leafy plant growth. It greens up grass and gets plants going in the spring. It leaches out of the soil quickly and must be frequently replaced. Too much nitrogen, though, can cause soft, weak growth, susceptible to disease and insect pests. It can also inhibit flower and fruit production on tomatoes, beans, peppers, and cucurbits. Add nitrogen in the spring before planting seeds.
Phosphorus is essential for healthy root development, as well as fruit and seed production. Phosphorus remains in the soil for a long time and is best applied in the fall. Phosphorus, whether as an organic amendment or a fertilizer, can pollute lakes, rivers, and streams, where it encourages excessive algae growth. This growth reduces the oxygen available in the water, potentially killing other aquatic life. Apply phosphorus only if your soil is deficient.
Highly soluble potassium boosts plant growth and encourages disease resistance. It must be replaced occasionally if your soil is deficient.
Soil pH refers to how alkaline or acidic your soil is. The pH scale ranges from 0 to 14, with 7 being neutral. Most soils have a pH level between 4, which is very acidic, to 10, which is very alkaline. The reason soil pH matters is because minerals in the soil are less available to plants in acidic or alkaline soils. Chlorosis, for example, occurs in alkaline soils because plants can’t access iron in the soil. Most vegetable plants grow best in a soil pH between 6.5 and 7.0.
What Soil Type Do You Have?
Before you attempt to grow a thing, it’s essential that you assess your soil’s condition. You can probably tell whether you have sand, clay, silt, or loam just by handling the soil, but determining its nutrient level and soil pH requires a soil test. The simplest and least expensive way to administer a soil test is to conduct one yourself. Test kits are available at garden centers and nurseries. For more detailed results, contact a private lab or university extension for a soil test kit. Follow the packaging directions and mail the soil test to the lab. Within a few weeks, you’ll receive a detailed analysis of your soil’s structure, fertility, and pH level, along with recommendations for improving your soil. Think a soil test is too much trouble? Soil tests typically cost less than $20, and the information you glean is invaluable in improving your soil. The small cost more than pays off in better harvests later.
Once you know what you’re working with, it’s time to develop a strategy for improving your soil through soil amendments. Some amendments are appropriate for almost all soil types, while others solve very specific problems. Here’s a rundown of the most common soil amendments available:
Composted manure: Composted chicken, cow, sheep, rabbit, or alpaca manure is one of the most valuable amendments you can find. It’s free if you have livestock, and many farmers give it away if you haul it. Manure adds nitrogen to the soil and also improves soil structure and drainage. However, manure has a few potential drawbacks. First, raw manure can carry human pathogens and has been known to cause serious illness. Never apply raw manure to the vegetable garden. Raw manure is also very high in nitrogen and can burn plants—another reason to avoid it. Horse manure is notorious for carrying weed seeds, although thorough composting can reduce this problem as well. Spread composed manure in the fall or early spring.
Compost: Like manure, compost adds nitrogen to the soil and improves soil texture. Unless you have a lot of composting material, it’s hard to make enough to fertilize a large garden. After your own composting bin, your next best bet is to order a bulk load from a landscaping firm. Bags of compost are convenient but expensive. When buying compost, avoid those that contain municipal sludge. Some research shows that these composts might contain heavy metals and other toxins you don’t want in the vegetable garden.
Peat moss: Peat moss is harvested from ancient northern bogs. This soil amendment is somewhat acidic and is excellent for lightening heavy clay soils or improving moisture retention in sandy ones. It has no nutrients, so be sure to use it with nutrient-rich amendments. Peat moss is also very dry. Spread it out in the garden and soak it well with a hose before you dig it in. Be aware that peat moss is a non-renewable resource, and some researchers fear that depleting peat bogs might contribute to global warming. It’s best to only use peat moss as an occasional soil amendment or for establishing new beds.
In addition to manure and compost as nitrogen sources, you can also try cottonseed meal or bat guano. Bone meal and horn meal are often used as a phosphorus source. Dry leaves and wood ashes add potassium to the soil. Use wood ashes with caution, though, because it will raise the soil pH.
Speaking of soil pH, a common question is how to raise or lower soil pH. To raise the soil pH, add dolomitic lime at a rate of five to ten pounds per 100 square feet of soil. Sandy soils need less; clay soils need more. Recommendations from a soil test analysis are the most reliable method for determining dosage. Lowering the pH of alkaline soils is trickier. Some gardeners rely on calcium sulfate or ground sulfur, but too much can damage the soil, and the alkaline-lowering results are still only temporary. Instead, dig acid-containing amendments, including sawdust, peat moss, pine needles, or old leaves, into the garden annually.